On iOS, the footer appears but subsequent navigation to different page and back makes footer invisible.
### Steps to reproduce issue on Windows
1. Create a Listview without any footer
2. Add a view to the footer myLV.Footer = myGrid in OnAppearing;
3. Navigate to another page.
4. Back to Listview page.
### Expected Results
The footer should remain visible.
### Actual Results
The footer is not visible after navigating back to ListView page.
### Version info
Reproduced with Forms 2.3.4.270 and Forms 2.3.5.256-pre1

Just based on what you've stated, it seems like this doesn't occur for me using the a very basic page with a ListView where its footer contains a Grid (the Grid contains only a Label for visibility confirmation). Footer = Grid is done in OnAppearing. There wasn't an apparent change regardless of whether or not the ListView needed to scroll.
Would you mind uploading a reproduction so we can see the exact code you're using in case there are any differences in particular, along with what device/emulator you're attempting to run the app on? Thanks in advance!